The present invention relates generally to industrial controllers for controlling equipment and processes and, in particular, to an industrial control system using a network and providing reduced jitter in control signals transmitted on the network and improved network utilization.
Industrial control systems are special-purpose computers used for controlling industrial processes or manufacturing equipment. Under the direction of a stored program, a programmable controller examines a series of inputs reflecting the status of the controlled process or equipment and changes outputs effecting the control of the process or equipment. The inputs and outputs may be binary (i.e., “on” or “off”) or, alternatively, analog, the latter taking on a continuous range of values. The binary inputs and outputs may be represented by single bits of data; the analog inputs and outputs may be represented by multiple-bit data words. The control program may have portions executed cyclically by the programmable controller to correspond with repeated reading and writing of input and output data
The input and output data normally is obtained from one or more input or output (I/O) modules which collect data from the controlled process or machine, and provide data from the programmable controller to the controlled process or machine. Because the various control points of a process or machine are often spatially distributed about a factory or manufacturing facility, the I/O modules may be connected to the programmable controller by one or more communication networks which transmit data among different control elements connected to the network as discrete data packets.
Network connections between elements of the industrial control system provide great flexibility in interconnecting these control elements. A single conductor may be routed among various elements and new elements added by simply adding an additional network tap. Network hubs, switches, and routers allow arbitrarily complex networks to be readily created permitting great flexibility in constructing and adapting the network topology.
In this respect, each of the control elements of the industrial control system may incorporate or be associated with a network adapter. For example, the I/O modules may be collected in a rack associated with a network adapter, the latter which cyclically collects data from the I/O modules and periodically transmits this collected data on the network to the programmable controller and periodically receives corresponding data from the programmable controller for outputting to the controlled process or equipment. Likewise, a network adapter may be associated with the programmable controller.
The communication networks used in industrial control systems are characterized by being highly reliable and by delivering data with a minimal and well-defined delay, as is required for real-time control. A number of different communication networks are commonly used in the industrial controller art including but not limited to: ControlNet™, DeviceNet™, and EtherNetIP™ whose specifications are published and whose protocols are used broadly by a number of manufacturers and suppliers. These communication networks, and particular implementations of these communication networks, differ from one another in physical aspects, for example, the type of media (e.g., co-axial cable, twisted pair, light fiber, etc.), the protocols of its operation, (e.g., baud rate, number of channels, word transmission size, use of connected messaging, etc.) and how the data is formatted and how it is collected into standard messages.
A well-known protocol for communication networks used for industrial control is the “connected messaging” protocol listed above. As is understood in the art, connected messaging establishes a logical connection between two control elements on a network (e.g. the network adapter of the programmable controller and the I/O adapter) which pre-allocates network bandwidth and buffer space at the control elements to ensure predictable and timely transmission of the data on the network without collisions and other unpredictable network delays. This may be contrasted to unconnected messaging systems where changes in network traffic can unpredictably affect the communication of messages.
The complexity of networks used for industrial control has increased, not only with respect to the number of network nodes (e.g. control points) but also with respect to the need for complex network topologies, including network hubs, bridges, routers and the like, needed to connect the control points to the network. This complexity can present problems of having adequate network bandwidth (a measure of the information capacity of the network in a given unit of time) while providing low transmission delay and jitter, the latter a measurement of the variability in the arrival time of data. Low delay and jitter are important to precise control where coordination and execution order of timed elements can be critical.